This invention relates to novel unitary, storable, heat-curable epoxy resin systems and the semi-latent curing agents incorporated therein. It also relates to the process for heat-curing epoxy resin systems containing these curing agents to produce insoluble, infusible cured resin products. It further relates to the process for preparing these curing agents by reacting piperazine with polycarboxylic acids to form amine salts.
The curing agents employed in the formulation of epoxy resins have a significant influence on the physical and chemical properties of the cured resins and the shelf or pot life of the compositions. They may be classified as non-latent, semi-latent, or latent curing agents or co-curing agents (i.e. accelerators).
As used herein, the term "shelf life" or "pot life" refers to the resin system's storage stability and working life, i.e. the time that it remains uncured and, in the case of liquid resins, free-flowing.
As used herein, the term "semi-latent" curing agents refers to agents which will be essentially unreactive with the epoxy resins for several months at room temperature and which will serve to cure (harden) the resins rapidly at moderately elevated temperatures. In contrast, "non-latent" curing agents cure epoxy resins rapidly at room temperature, whereas "latent" curing agents are essentially unreactive for an indefinite period at room temperature and cure the resins only at elevated temperatures and then not always rapidly.
Unitary systems (one package) are desirable because they eliminate the additional mixing step before using, avoid the possibility that the consumer may use the incorrect amount of curing agent, and further avoid the need for complex packaging. Previous attempts to provide unitary systems have involved the use of aromatic amines as latent curing agents; they have an excellent shelf life but require high curing temperatures and long cure times. Dicyandiamide (often called Dicy) and dihydrazides have also been used as latent curing agents; unfortunately these curing agents also suffer from a slow cure at elevated temperatures. Dicy, the most desirable because it gives the best strength properties, has a shelf life of over 1 year, but it requires 1 hour to cure at 170.degree. C. and does not cure at all at 120.degree. C.
Various amine salts have been suggested for use in epoxy resin systems; they have functioned as non-latent curing agents and co-curing agents, as well as latent or semi-latent curing agents. Those useful as semi-latent or latent curing agents include amine-boron trifluoride complexes (U.S. Pat. No. 2,717,885), nylon salts (Japan. Pat. No. 66/7458), imidazole salts of monocarboxylic acid (U.S. Pat. No. 3,356,645), and imidazole salts of polycarboxylic acids or anhydrides (U.S. Pat. No. 3,746,686).
Amine-boron trifluoride complexes, e.g. ethylamine-boron trifluoride, are well known. They have an excellent shelf life but, like Dicy and the dihydrazides, they are slow to cure epoxy resin systems even at elevated temperatures, and the salts themselves are hygroscopic and hydrolyze in moist air. Epoxy systems containing nylon salts are stable for several months; however, a system containing the nylon salts of diethylenetriamine and adipic acid required 4-5 hours to cure at 130.degree. C. and the adhesive strength of the resulting bond was only 250 kg/cm.sup.2. Epoxy systems containing the imidazole salts of monocarboxylic acids, e.g. 2-ethyl-4-methylimidazole acetate, benzimidazole acetate, or imidazole lactate, have a relatively long shelf life; however most are hygroscopic. Imidazole salts of polycarboxylic acids or anhydrides, e.g. imidazole phthalate monohydrate and monoimidazole adipate, are somewhat less hygroscopic than the salts of the monocarboxylic acid; however, epoxy systems containing them were stable for only about 7 days at room temperature and then only when low levels of the salts were used.
It is an object of this invention to provide novel unitary, storable, heat-curable epoxy resin systems containing semi-latent curing agents (hereafter simply referred to as curing agents) and process for curing these systems at moderately elevated temperatures to give high performance products.